The subject matter disclosed herein relates to furnaces. More specifically, the present disclosure relates to a coupling between heat exchanger components of a multi-position gas furnace.
In a typical furnace, a fuel, for example, natural gas, is combusted in a burner. The byproduct of combustion, called flue gas, is routed through one or more heat exchangers which extract the heat therefrom. In a condensing gas furnace there are two types of heat exchangers: a primary heat exchanger (PHX) and a secondary or condensing heat exchanger (CHX). The PHX accounts for most of the efficiency of the furnace by reducing the heat of the flue gas from the flame temperature to a temperature well above the dew point temperature of the water in the flue gas. The flue gas heats the surface of the PHX and air is blown across the exterior of the PHX thus removing heat from the PHX by convection. Efficiency is measured by the amount of heat energy that is transferred out of the flue gas compared to the amount of heat energy that remains in the flue gas as it leaves the heat exchanger. It can be determined roughly by knowing how much air and gas enters and is burned in the PHX, and the temperature of the gas leaving the PHX. The CHX makes up the remainder of the furnace efficiency by reducing the flue gas temperature below the dew point of the flue gas and thus taking advantage of the latent heat from the water byproduct of combustion.
The PHX and CHX are connected via a coupling box, with flow from the PHX proceeding through the coupling box and into the CHX. Without adequate distribution of the flue gas through the coupling box, localized spots on parts of the coupling box may fall below the dew point temperature of the flue gas causing there to be localized condensation prior to the flue gas entering the CHX tubes. Baffles and other means to distribute the flue gas to all parts of the coupling box will increase internal pressure drop, which requires a larger combustion blower. The moisture deposits outside of the CHX reduce the effectiveness of the CHX and also can cause corrosion of components of the coupling box and/or PHX. Further, there is a desire to reduce the size of the furnace, if possible, to reduce cost and household space occupied by the furnace. If a furnace requires a larger combustion blower, the size of the blower will limit how much the overall size of the furnace can be reduced.
The furnace being a multiposition appliance must perform its intended function in the 4 major positions of upflow, laid horizontally on its left side, laid horizontally on its right side, and upside down, or downflow. If orientation was limited to a single position the CHX could be designed to provide a natural slope that would allow condensate formed in the tubes to drain by gravity in the direction of the slope. However, in order to accommodate multiple positions (that is more than 2 positions) the CHX must be designed to be level, with no intended slope within the appliance. Generally as the furnace operates and the combustion blower is operating to pull the gas through the CHX, it will provide enough flow velocity to assist with water drainage to the intended front of the furnace. When the furnace shuts off, there is no air flow through the CHX, and water remaining in the tubes will migrate to either the inlet or the outlet. If the furnace is installed in a residence in a way that the CHX tubes slope gradually backwards in the unit, gravity will tend to lead the remaining water to the back of the furnace where it can pool and cause a leak, or corrosion on parts that are not intended to handle condensate.